Every year ~ 500,000 febrile infants = 60 days of age present to US emergency departments (ED). 6-10% of these febrile infants will have invasive bacterial infections (bacteremia, urinary tract infections or meningitis). Current approaches for the evaluation of young febrile infants are suboptimal because they include frequent invasive procedures, overuse of empirical antibiotics and unnecessary hospitalizations, which can lead to iatrogenic complications and have substantial cost implications. The primary cause of these problems is lack of highly accurate and short turn-around testing to reliably distinguish young febrile infants with and without bacterial infections. The long-term objective of our research is t investigate whole genome RNA expression profiles to define RNA biosignatures that allow precise diagnosis of isolated bacterial infections, isolated viral infections and bacterial-viral c-infections in febrile infants = 60 days of age. Our specific aims are to (1) define the RNA biosignatures in febrile infants = 60 days of age with isolated bacterial infections, isolated vira infections and viral-bacterial co-infections; (2) demonstrate the stability of the above defined RNA biosignatures over a 24-72 hour time period; and (3) validate the RNA biosignatures on a novel, PCR-based platform that has a rapid (2 - 4 hour) turnaround time. We will conduct a prospective, multi-center, cross-sectional study of febrile infants = 60 days of age who are being evaluated for bacterial infections. After obtaining informed consent from the guardian we will collect 2 ml of whole blood (1 ml for RNA expression analysis; 1 ml for comprehensive viral studies) and 1 nasopharyngeal (NP) swab for a comprehensive respiratory viral diagnosis. Additionally, in a selected group of febrile infants who are either hospitalized from the ED or those who return to the ED, we will obtain a second 1 ml blood sample for sequential RNA biosignature analysis at 24 - 72 hours after the initial sample collection. Using an aliquot of blood from the same sample we will validate RNA biosignatures on the PCR-based platform. Addressing the optimal evaluation and management of the febrile infant will assure that this vulnerable population of children have the chance to achieve their full potential for healthy and productive lives, free from disease or disability and is consistent with the mission of NICHD.